MBIST

MBIST

The ever increasing size and number of memories in the Systems on Chip has presented the designers and test engineers with a challenge for huge number of functional or ATPG patterns for verification of memory functionality. So, to test the memory functionality either functionally or through ATPG requires huge test time, and hence, huge test cost. It is almost impossible in such scenario to verify memory functionality fully. Thus, the designers are left with only one way; i.e. to verify memory functionality through BIST (Built-In Self Test) functionality.

BIST is an inbuilt testing circuitry within a software/hardware module. We just need to trigger the circuitry from outside. This circuitry, then, runs the inbuilt patterns/algorithms and returns if the module is working properly. This, being inbuilt does not need to be supplied with patterns from outside. Also, since, this is within a module, hence, we can take the modular approach for testing which reduces run time significantly.

The built-in self test employed for memories is known as MBIST (Memory Built-In Self Test). Like other BIST logic, MBIST logic is inbuilt within memory only. The MBIST logic may be capable of running several algorithms to verify memory functionality and test for memory faults specifically designed and optimized for these.

There is usually a wrapper around memory, known as ‘memory collar’ that is used to select between functional inputs and test inputs based upon MBIST/functional mode selection bit. It interfaces the memory with on-chip logic and MBIST controller. The MBIST controller indicates the start of MBIST with a select input. The memory, then, starts the BIST algorithms and provides the test output to the controller. The controller compares this output with the reference output and indicates if the MBIST has passed or failed. There can be one controller for several memories. Also, memories can share the collar depending upon the test time requirement and type of memories.

Advantages of MBIST: There are several advantages of MBIST insertion over functional/at speed testing such as:

• It allows for robust testing of memories 
•  Reduced test time 
•  All the memories of the design can be tested in parallel 
•  Lesser test cost

Disadvantages of MBIST: Inspite of many advantage of MBIST, there is only one remarkable limitation. Insertion of MBIST causes increase in area. However, this increase in area is very small in comparison to the benefits it provides.